1. Field of the Invention
The present invention relates to measurement techniques used in semiconductor processing and fabrication. More precisely, the present invention relates to nondestructive measurement techniques for determining implant dose in semiconductor devices.
2. Description of the Related Art
In semiconductor processing and fabrication arts, considerable investment has been made in developing high resolution measurement devices and techniques for evaluating sample parameters. Many various types of measurements are performed in the semiconductor industry where very thin films, such as oxides, metals or dielectrics, are deposited on semiconductor substrates such as silicon. Non-destructive techniques are particularly useful for evaluating thickness, impurities and index of refraction characteristics of the films to insure high yields during fabrication.
One type of information that is particularly useful in semiconductor fabrication is information relating to the dose and profile of ion implantation of dopants such as arsenic (As), phosphorus (P), and boron (B). One conventional technique for monitoring implant dose is ellipsometry which measures damage that results from implantation. In another conventional technique, implant doses are monitored using a Faraday cup or cage in the implanter. Ellipsometry and Faraday cup techniques are nondestructive tests. In contrast, implant profiles are usually measured using a secondary ion mass spectometry (SIMS) laboratory set-up outside the fab.
Conventional ellipsometric techniques are non-contact, optical measurement techniques. A probe beam having a known polarization state is directed to interact with a sample, resulting in a change in the polarization state that is monitored by measuring a selected parameter of the reflected probe beam. For example, as the thickness of a film varies, the intensity of the reflected probe beam also varies due to the variation in interference effects created at the interface between the thin film and the substrate. Also the thickness of a thin film changes the polarization state which occurs when the probe beam is reflected off the sample surface. By monitoring either the change in intensity of the reflected probe beam or the change in polarization state (ellipsometry), information about the thin film is derived. An analyzer determines the polarization state of the beam following the beam interaction with the sample. The change in polarization state of the beam caused by the interaction with the sample varies as a function of the sample parameters. Measurement of the change supplies data used in analysis of the sample. In a typical measurement scenario, the azimuthal angle of the polarizing or analyzing elements are varied to obtain multiple measurements. Usage of multiple measurements increases accuracy. One approach for improving accuracy is to perform measurements at several different wavelengths using a spectrophotometer that performs multiple-wavelength interferometric-type measurements. Another approach for improving accuracy is to measure at a number of different incidence angles of the probe beam.
Faraday cages trap and measure ion beam current during implanting while blocking electrons that might accompany the ion beam. Faraday cages do not measure neutral atoms in the ion beam. Since neutralized atoms have essentially the same energy as the ions and are individually equivalent in terms of implantation dose, if significant neutralization of the beam takes place, the Faraday cage reading expresses a false measure of the implantation current. The Faraday cage determines a sensed beam intensity of an ion beam at the target chamber. An implant controller determines beam current from the sensed beam intensity by accounting for both charge stripping and charge neutralization of ions within the ion beam caused by interactions between the ions that make up the beam and residual gas molecules encountered by the beam along the beam path to the target.
Secondary ion mass spectroscopy (SIMS) is a prevalent analytical method in the microelectronics industry today for characterizing materials present in semiconductor processes. In SIMS, a sample to be studied is bombarded with a primary beam of energetic ions. The ions sputter away ionized particles, for example secondary ions, from the surface of the sample. The secondary ions are directed into a mass spectrometer which identifies the ions as a function of a mass to charge ratio. Continued sputtering dislodges particles and secondary ions located below the surface of the sample. Thus, SIMS permits analysis of elements embedded within the sample as a function of sample depth. For example, SIMS data may express that 10.sup.19 atoms of boron per cm.sup.2 reside at the surface of a silicon sample while 10.sup.19 atoms of boron per cm.sup.2 reside one micron below the surface. SIMS is presently used to measure the amount of material embedded within a thin film. However, SIMS does not meet the requirements of being an inexpensive, non-destructive technique requiring little maintenance, having the ability to serve as a portable monitoring station.
Although SIMS depth resolution, lateral resolution, and sensitivity continue to improve as technology advances, several drawbacks are inherent with SIMS measurements. The biggest drawback is SIMS character as a destructive technique. SIMS sputters away layer after layer of material from the surface of the sample. Thus SIMS is not feasible for usage as a bench-top process control station for monitoring the amount of material embedded within a substrate. Furthermore, SIMS is a very bulky, complex, expensive method that employs complicated, maintenance-intensive machinery. For instance, SIMS instruments typically occupy an entire room in a midsized laboratory and include several vacuum pumps, valves, powerful magnets, energy filters, ion sources, and complex data analysis tools.
What is needed is an analytical method that measures the implant dose nondestructively.